PROJECTSUMMARYABSTRACT Duringaninfection,animalsexhibitadaptivechangesinbehaviorandphysiologyaimedatincreasingsurvival. Although many causes of acute infection exist, a similar set of stereotyped symptoms occur, which includes increasedbodytemperatureorfever,decreasedappetiteandincreasedlethargy.Bothwarm-andcold-blooded animals generate a fever in response infection suggesting that fever circuits are hard-wired and highly conserved,yetexactlyhowthenervoussystemaltersbodytemperatureandassociatedbehaviorinresponseto infectionremainsunknown.Wehaveidentifiedapopulationofneuronsinthepreopticareaofthehypothalamus that are highly activated following administration of inflammatory lipopolysaccharides (LPS). Due to the close proximitybetween the organum vasculosum of the laminae terminalis (OVLT), where inflammatory cytokines enterthebraintoaffectnearbycells,andneuronsofthepreotpicarearegulatingnormalbodytemperature,and ourpreliminarydata,weproposethatthesenewlyidentifiedLPS-sensitiveneuronscontrolfeverinitiationduring animmuneresponse.Wewillusechemogeneticactivationandcellablationapproachestodemonstratethatthis populationplaysaroleinincreasingbodytemperatureandinaffectingotherfever-associatedbehaviorsupon LPSinjection.Further,wehaverecentlydevelopednewapproachesformolecularcharacterizationofgenetically defined cell populations in situ using single-cell RNA sequencing (scRNA-seq) and multiplex, error-robust, fluorescentinsituhybridization(MERFISH)togenerateaspatially-resolvedandfunctionally-awareatlasofthe preopticarea.Wewillapplyasimilarstrategytocharacterizefever-inducingneuronsaswellassurroundingnon- neuronal cell types thatare likely to play a role infevergeneration throughparacrine mechanisms.Finally, we proposetouseviral-mediatedtracingandfunctionaltoolstodeterminethedirectandindirectcircuitmechanisms by which LPS-sensitive neurons and their targets exert control over body temperature and fever-related behaviors. Our data will lead to a molecular and functional characterization of LPS-sensitive neurons in the preopticareaandtoabetterunderstandingofhowinflammatorysicknesssymptoms,suchasfeverandrelated behavioral changes, are regulated in the brain. These efforts have direct implications for understanding the mechanismsunderlying human sickness,and may inform newtherapeutic strategiesforthetreatment offever andassociatedsymptoms.